1. Field of the Invention
This invention relates to the treatment of waste water to remove the B.O.D. More especially, the invention is concerned with a process and which permits the employment of a single vessel for carrying out biological reaction and the clarification by settling of the solids from the biological reaction.
2. Description of the Prior Art
U.S. Pat. No. 4,192,740, the disclosure of which is hereby incorporated by reference, as a basis for explaining the invention claimed, describes conventional methods of purification and biological treatment of waste water from municipal and industrial sources prior to discharge into natural water systems. It also describes prior proposals for modifying the conventional treatment as disclosed, for example, in U.S. Pat. No. 3,476,682 Albersmeyer and U.S. Pat. No. 3,983,031 Kirk, as well as the oxygen-dissolving device of Speece described in U.S. Pat. Nos. 3,643,403 and 3,804,255.
The invention of the prior U.S. Pat. No. 4,192,740 is directed to conducting both the biological treatment and the clarification of biologically treated water in a single vessel. Several embodiments of the invention are disclosed, one employing a device in which oxygen is dissolved in the mixed liquor and waste water influent in a conical chamber within the vessel, and the oxygenated mixed liquor recycled from the bottom of the conical chamber through a biological reaction zone and into the top of the chamber. In other forms of the invention disclosed, the recycle stream of oxygenated mixed liquor and influent is injected into the bottom of the biological reaction zone and a recycle stream withdrawn from that zone. In all cases the process is directed to careful monitoring of the oxygen, in the biological reaction zone, and adjusting the supply of oxygen in the incoming recycle stream to meet the oxygen demand of the biodegrading organisms and avoid the occurrence of undissolved oxygen in the form of gas bubbles in the biological reaction or clarification zones.
A process, according to U.S. Pat. No. 4,192,740 is carried out as follows.
Waste water is continuously passed through a single treating enclosure open to the atmosphere containing waste-degrading microorganisms, to which oxygen is added to sustain the microorganisms and from which the clarified effluent is continuously overflowed and from which excess sludge and gases are removed.
In starting up the process, there is initially established:
(a) in a lower part of the enclosure a biological reaction zone containing mixed liquor containing said microorganisms and in which a biological reaction to degrade the waste is conducted,
(b) in an upper part of the enclosure a clarification zone in which clarified liquid rises and overflows,
(c) between the reaction and clarification zones a transition zone in which the liquid of the mixed liquor rises and the solids settle.
Then, after these conditions have been established, there are then carried out continuously, the following steps.
A recycle stream of mixed liquor from the reaction zone is withdrawn and conducted through an oxygen-dissolving device disposed outside the reaction zone, influent waste water added to it, oxygen dissolved in the stream, and the supplemented stream injected into a lower part of the reaction zone remote from the vicinity of withdrawal.
The waste water is conducted into the recycle stream at a variable rate within a range related to the depth and surface area of the enclosure to provide a residence time within the reaction zone effective for the biodegradation of the waste and for the formation and settling of biological floc.
Oxygen is added to the recycle stream at a rate to provide an oxygen concentration within a controlled range below the saturation level of oxygen in the liquid effective to meet the oxygen demand of the organisms and maintained in contact with the liquid in a contact zone of the stream for a time and under a pressure such that the oxygen is dissolved in the liquid.
The overall flow rate of the recycle stream is controlled to a substantially constant rate several times that of the incoming waste water effective to provide:
(d) for dissolving the oxygen which is added to the recycle stream,
(e) an amount of dilution of the recycle stream entering the reaction zone effective to prevent the oxygen coming out of solution at an upper part of the reaction zone.
The flow of the recycle stream entering the reaction zone is distributed to reach a substantial area of a lower part thereof:
(f) to provide a wide spread direct flow through the reaction zone, from the vicinity of injection to the vicinity of withdrawal, whereby there is controlled agitation effective to keep the solids dispersed, and good access of the organisms to the biodegradeable waste,
(g) and to provide at an intermediate level of the enclosure, an upward velocity of the mixed liquor less than the settling rate of the solids, whereby there is maintained in the enclosure separate reaction and clarification zones intervened by a transition zone.
The concentration of dissolved oxygen in the reaction zone is continuously monitored to determine variations thereof, resulting from variations in the flow rate and concentration therein of waste.
The rate of addition of the oxygen to the recycle stream is periodically adjusted in response to variations in the oxygen concentration in the reaction zone to maintain the concentration within the controlled range and at a level where there is substantially avoided effervescence that would lead to gas bubbles rising into the clarification zone.
The effluent is continuously withdrawn from the clarification zone to keep pace with the influent waste water. And, continually excess sludge is removed from the reaction zone and carbon dioxide removed from the mixed liquor.
The parent applications disclose various embodiments of the invention including several different types of apparatus for carrying out the process.